Dendrochronology - Huh?
/Most of us know that you can tell the age of a tree by counting the number of rings in its trunk. Most of us also know that the width of the individual rings in the trunk give an indication of how amenable that year was for growth – a wider ring indicates that the year was probably wetter and had a longer growing season than a dry, cold year, for example. That pretty much sums up my entire knowledge about tree dating.
Tree rings and what they mean. Photo: Darach Croft
It turns out, however, that there is a whole lot more that can be gleaned by studying tree rings than how old a particular tree is. In fact, there is an entire field of science called dendrochronology devoted to the subject, about which we learned a great deal while visiting the Ancient Bristlecone Pine Forest.
An aside here – I’m a pretty nerdy guy, but even so, I have to admit that some visitor center exhibits just don’t elicit much enthusiasm from me. In particular, while there must be a great many people who find the exhibits devoted to the geological history of an area to be interesting, they don’t exactly captivate me. That being true, I was pleasantly surprised at how fascinating I found the study of tree rings to be. I’ll try to explain why.
First, there’s one major misconception I’ve always had – I thought the tree had to be cut down to count its rings. “Hey Vern, that bristlecone pine over there looks really old. Let’s cut it down and see just how old it is.” Instead of cutting the tree down, naturalists can extract a pencil-sized core from a tree using a tool called an increment borer, which is essentially a hollow drill bit with a handle. The drill bits range from 16” to over 3’ long. The increment borer is screwed into the tree, capturing a small core. Once the tool is backed out of the tree, the core sample can be pushed out of the borer, and can be used to view the tree rings extending from the bark to the center of the tree. Because the hole is so small, the tree heals itself, resulting in no long-term damage.
So, my next question was “why is knowing the age of the tree useful?”. The answer to this question is why I found the science of dendrochronology so interesting.
We all know that the width of each ring indicates how favorable a particular year was for growth. So if we look at a core sample for a tree that is 100, 200, or in the case of a bristlecone pine, 3,000 years old, and starting with the ring closest to the bark, we can count the rings backwards to determine which years were wetter and/or warmer than other years.
This is mildly interesting, as scientists can determine when droughts occurred, or what years were wetter or warmer than others. More importantly is the fact that all the trees in the same general area will have the same pattern to their rings. For example, if 1256 was a drought year, but 1257-1260 were relatively wetter years – the rings in all the trees will vary accordingly. For argument’s sake, let’s say the rings in our core sample were 2mm, 5mm, 8mm, 6mm and 6mm wide for those years. Knowing this, an archeologist who wants to date an ancient structure can take a core sample from a wooden beam supporting the structure and examine the pattern of ring widths. If the outer five rings of the beam have the pattern 2mm, 5mm, 8mm, 6mm and 6mm, then there’s a high likelihood that the tree was cut down in 1260.
A Typical Core Sample. Photo: Seattle Times
Courtesy of Open Oregon Education Resources
Andrew E. Douglass was the founder of dendrochronology. He was a Harvard-educated astronomer who was tasked in 1892 with building and operating the Lowell Observatory in Flagstaff, Arizona. While there, he theorized that since the growth of the trees in the area was limited by the available moisture in the arid southwest, measuring the tree rings from logs and stumps would reveal past climate trends. He hoped to find a correlation between sunspot activity and the Earth’s climate, and to prove this, he set about amassing and recording tree ring data. Since ponderosa pines typically live 300-400 years, and some as long as 700 years, over the next decade, he focused on this species of tree and compiled climate data spanning 450 years -the age of the oldest trees he could find. In 1927, he made another major breakthrough when he sampled several beams in a prehistoric ruin near Show Low, Arizona. The ring patterns in some of the younger beams matched those of the oldest ponderosa pines he had previously recorded, while the rings from the older beams extended the climate data back to around 900 CE.
Photos: Laboratory of Tree-Ring Research, Tucson
While Douglass never found a correlation between sunspot activity and climate, he did establish an entirely new field of science. He was also the first person to use an increment borer, making it possible to record a tree’s ring pattern without cutting it down.
Other researchers continued his work by taking core samples from a variety of other trees and compiling tree ring data from around the world. When enough overlapping tree ring patterns exist, a “master chronology” can be created. In particular, the master chronology for bristlecone pines extends almost 9,000 years back to around 7,000 BCE, while the master chronology for the Hohenheim oak in Europe goes back 12,500 years, towards the end of the Stone Age and about the time that man first started domesticating animals.
And lastly, some related trivia:
Carbon dating calibration. Carbon 14 dating has long been used to date organic substances as old as 50,000 years, but the accuracy can be off by decades or even centuries. Dendrochronology can be used to calibrate the carbon dating instruments.
The tree rings in European trees indicate there was a long, exceptionally dry period between 250 and 550 CE, which corresponds to the demise of the Roman Empire. Historians surmise that this may have resulted in poor and failing crops and food supplies, making it difficult for the Romans to continue supporting and controlling their far-flung empire and contributing to its downfall.
Dendrochronology can also be used to date ancient shipwrecks by examining core samples of the ship’s timbers.
A cross section of giant sequoia shows fire scars. PHOTO: University of Arizona
Wildfire research. Trees often survive wildfires, but the damage done to the tree is recorded in the tree’s rings. By examining the scarring in the tree rings, researchers can determine the frequency and severity of past wildfires.
Viking settlements. By examining the trees that were felled to build the Viking settlement at L’Anse aux Meadows in Newfoundland, archeologists were able to pinpoint the exact year it was built: 1021 CE.
Historical artifact dating. Almost anything made of wood that is large enough to have at least ten years of tree growth can be dated. For example, a study of an alleged 18th-century portrait of Mary, Queen of Scots, revealed the painting's wooden panel dated from the 16th century, reclassifying it as an original.
Flood records. Tree rings record periods of heavy rains as well as droughts. An example is the catastrophic Great Famine of 1315-1317 – one of Europe’s worst famines ever. Historical records indicate there was flooding, mass crop failure, mass starvation, and even instances of cannibalism. Tree ring data indicates those years were some of the wettest in Europe’s history.
Maybe it’s just my inherent nerdiness, but I think this is all pretty cool. See you next time...